British forces became embroiled in the fight to wrest control of Basra from Iranian-backed militias yesterday, as a senior US military adviser accused Gordon Brown of failing "as an ally" in his desire for a hasty withdrawal of troops.

British forces became embroiled in the fight to wrest control of Basra from Iranian-backed militias yesterday, as a senior US military adviser accused Gordon Brown of failing "as an ally" in his desire for a hasty withdrawal of troops.

While Iraqi Army forces continued to struggle to subdue Mehdi Army militiamen, British troops became directly engaged for the first time. British artillery based at Basra airport pounded guerrilla positions while US and British warplanes took part in bombing and strafing runs.

The fighting, however, continued to spread to Baghdad and other Mehdi Army stronghold cities, prompting fears that it could undo hard-won security gains achieved by the year-long US "troop surge" in the capital.

Although British commanders in Basra still intend to play only a back-seat role, the deteriorating security picture nationwide prompted harsh comments from the principal architect of the surge strategy.

Mr Kagan, a fellow at the American Enterprise Institute think-tank, told The Sunday Telegraph: "British forces have an obligation to step up when needed and it sure looks here like they're needed.

"It is rather a watershed moment in the Anglo-American alliance. I understand that your Prime Minister has already said that the special relationship is over. There's an issue here of fulfilling your obligations as an ally, freely undertaken."

His fellow surge architect, retired US general Jack Keane, also voiced doubts that the Iraqi security forces would be able to pacify Basra unassisted. "There are about 8,000 armed militiamen with a stranglehold on the people of Basra. The situation in Basra has deteriorated since the British pulled out."

Their comments are likely to embarrass Downing Street and anger British commanders in Basra, who have insisted their policy of scaling down their presence is to encourage Iraqi security forces to take the lead. Senior officers also said that the coalition command in Baghdad approved their plans.

However, both surge advisors have the ear of the White House and the Pentagon.

Mindful of US unease over Basra, Des Browne, the Defence Secretary, will signal this week that there will be no withdrawal of UK troops from Iraq this spring.

He will tell the Commons that "all options remain under review", but government sources said it was accepted within the military that any troop withdrawal at this time would be "presentationally unacceptable".